This invention relates generally to rotation sensors and particularly to fiber optic rotation sensors. Still more particularly, this invention relates to a fiber optic rotation sensing system suitable for aircraft navigation using differential frequency propagation to compensate for the Sagnac phase shift in a rotating fiber optic loop to measure the rotation rate and angular displacement.
A fiber optic ring interferometer typically comprises a loop of fiber optic material having counter-propagating light waves therein. After traversing the loop, the counter-propagating waves are combined so that they constructively or destructively interfere to form an optical output signal. The intensity of the optical output signal varies as a function of the type and amount of interference, which is dependent upon the relative phase of the counter-propagating waves.
Fiber optic ring interferometers have proven to be particularly useful for rotation sensing. Rotation of the loop creates relative phase difference between the counter-propagating waves, in accordance with the well known Sagnac effect, with the amount of phase difference being a function of the angular velocity of the loop. The optical output signal produced by the interference of the counter-propagating waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing the optical output signal to determine the rotation rate.
In order to be suitable for inertial navigation applications, a rotation sensor must have a very wide dynamic range. The rotation sensor must be capable of detecting rotation rates as low as 0.01 degrees per hour and as high as 1,000 degrees per second. The ratio of the upper limit lower limits to be measured is approximately 10.sup.9.
The output of an open loop fiber optic gyroscope is a sinusoidal waveform having a phase shift relative to a reference for indicating a detected rate of rotation. The sinusoid is nonlinear and not single valued, which present difficulties in obtaining accurate measurements. The amplitude also fluctuates because it is dependent upon several parameters that may fluctuate.
All fiber gyroscopes having low bias levels and high sensitivity to rotations have been constructed. However, the dynamic ranges of previous all fiber gyroscopes are very limited and their outputs are nonlinear with respect to rotation rate.